Theme: Twenty Thirteen Mobile Sliding Menu Doesn’t Bump Content Down

You’ve got a static height set on a block-level element:

style.css:806

/**
 * 4.1 Site Header
 * ----------------------------------------------------------------------------
 */

.site-header {
    position: relative;
    background: url(http://barkleyphoto.com/_wedding/wp-content/uploads/2014/05/cropped-header-background.jpg) no-repeat scroll top);
    height: 100px;
}

remove the height: 100px; and it should work like you expect.